The exemplary embodiments generally relate to xerographic marking and devices, and specifically relate to media handling, such as feeding, transport, and finishing.
Various systems and methods for automatic sheet registration, including sheet deskewing, are known in the art. One such system and method is described in U.S. Publication No. 20055/0263958 for Knierim et al. for controlling, correcting, or changing the orientation and position of sheets traveling in a sheet transport path. The sheets include those being printed in a reproduction apparatus, which may include sheets being fed for printing, sheets being re-circulated for second side (duplex) printing, and/or sheets being output to a stacker, finishers or other output or module.
The related art discussed below demonstrates the long-standing efforts in this technological art for more effective sheet registration for xerographic devices, such as printers, copiers, facsimile devices, scanners, and the like. The related art includes translation electronic registration (TELER or ELER) sheet deskewing and/or side registration systems, such as U.S. Pat. No. 6,575,458 to Williams et al., and U.S. Pat. No. 6,736,394 to Herrmann et al. In either ELER or TELER systems, initial or incoming sheet skew and position may be measured with a pair of lead edge sensors, and then two or more ELER or TELER drive rollers may be used to correct the skew and process direction position with an open loop control system in a known manner. The drive rollers have two independently driven, spaced apart, inboard and outboard nips. Some ELER systems use one servomotor for process direction correction and another motor (e.g., a stepper motor) for the differential actuation for skew correction, as variously shown in U.S. Pat. Nos. 6,575,458 and 6,535,268 to Williams et al. Other ELER systems have separate servo or stepper motors independently driving each of the two laterally spaced drive nips for process direction registration and sheet skew registration.
Many sheet transport systems including most TELER and ELER systems use a frictional force drive nip to impart velocity to a sheet. Typically, a nip consists of a motor driven elastomeric surface wheel or “drive roller” and a backup wheel or “idler roller” that is spring loaded against the drive roller to provide sufficient normal force for a normally non-slip drive of the sheet. A well known example of the drive roller surface is a urethane material. In contrast, the idler roller (wheel) is usually a hard substantially inelastic material (metal or hard plastic). The angular velocity of the drive nip has typically been measured with the encoder mounted on either the drive nip, or on the servo or stepper motor driving the drive roll directly or through a transmission as in a timing belt drive.
Many paper registration systems in printers use two drive nips (inboard and outboard nips) as part of the paper path delivering the sheet from an input location to an image transfer position, where an image is transferred to the sheet. In order for the image to be properly positioned on the sheet, the sheet position (in both process direction and skew) needs to be within defined, desired specifications, even though the arrival position of the sheet at the image transfer position may be downstream from the two variable speed drive nips or other paper registration system providing the sheet to image registration. Typically, the position of the sheet is measured at an input location and a desired sheet trajectory is calculated. From that desired sheet trajectory, the desired nip velocities are calculated. That is, the average of the two nips will determine the process direction position correction and the differential velocity of the two nips will determine the skew registration correction. The compliance of the compliant drive nip causes the sheet velocity to be different from the imposed velocity by the drive nip. The ratio of actual paper velocity to the imposed velocity is known as the drive ratio. This drive ratio error effect will cause that desired paper trajectory to differ from the actual paper trajectory. This can lead to significant output registration errors that are outside of the defined, desired specifications. As a result, the sheet may not be sufficiently accurately aligned or overlaid with one or more print images.
For printing in general, providing sheet skewing rotation and sheet registration while the sheet is being fed forward in the printer sheet path is a technical challenge, especially as the sheet path feeding speed for systems increases. Print sheets are typically flimsy paper or plastic imageable substrates of varying thickness, stiffness, frictions, surface coatings, sizes, masses, and with various humidity conditions. Sheets of some with these various characteristics are particularly susceptible to feeder slippage, wrinkling, or tearing, especially when subject to excessive accelerations, decelerations, drag forces, path bending, and the like.
In addition to sheet lateral registration based on deliberate skew inducement and removal and TELER systems, there are other sheet side-shifting lateral registration systems, in which the entire structure and mass of a carriage containing the two drive rollers, their opposing nip idlers, and the drive motors (unless splined drive telescopically connected) are axially side-shifted to side-shift the engaged sheet into lateral registration. However, even in such systems, the sheet lateral registration movement can be done during the same time as, and independently of, the sheet deskewing movement. These may also be broadly referred to as TELER systems. For example, see U.S. Pat. No. 5,094,442 to Kamprath et al.
In various sheet registration systems, the use of sheet position sensors, such as a charge-coupled device (CCD) multi-element linear strip array sensor, may be used in a feedback loop for slip compensation to ensure the sheet achieves the desired three-axis registration. Sheet registration systems are operated and controlled by appropriate operation of conventional control systems. It is well known to program and execute imaging, printing, paper handling, and other control functions and logic with software instructions for processors, as taught by numerous prior patents and commercial products. Such software may, of course, vary depending on the particular functions, software type, and processor or other computer devices used, and may alternatively be implemented partially or fully in hardware using standard logic circuits or other designs.
Many sheet transport nips consist of a compliant drive wheel and a non-compliant idler wheel that is spring-loaded against the drive wheel. The compliance of this drive nip is known to cause a forward velocity error. U.S. Publication No. 2005/0263958 for Knierim et al., describes the measurement of sheet velocity from encoded non-compliant idler wheels. The Knierim method measured the actual surface velocity of the sheet without errors introduced by the compliance. Testing has revealed that the Knierim measurement method has systematic errors. One observed systematic error is caused by sheet thickness variations.